I need some way of estimatinn the replacement cost of my house.
I'm in the NY area. I don't believe building costs are as high as $300 per square foot, even in NY

It depends what rebuilding costs are in your area. Call some contractors and ask over the phone. Ask how much it costs per square foot to rebuild in your area.

Your insurance agent should have a rough idea or give you an estimate.

Each insurance company uses a slightly different method. YOUR AGENT should be doing this for you!!

If you're trying to double check your agent, you can hire a local builder, to give you a written estimate - but keep in mind, that's not binding to the insurance company!

Generally, depending on location, it's probably going to be $200 to $300 per square foot, unless you have a special situation (like, it's a beachhouse, and you need to put it on pilings - or, the dining room has gilt gold moldings).